nix search uses NIX_PATH to determine where to find packages.
$ nix search -u podman
* nixos.podman (podman)
A program for managing pods, containers and container images
* nixpkgs.podman (podman)
A program for managing pods, containers and container images
$ NIX_PATH= nix search -u podman
error: no results for the given search term(s)!
$ echo $NIX_PATH
/home/jon/.nix-defexpr/channels:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
$ NIX_PATH=/home/jon/.nix-defexpr/channels:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos nix search -u podman
* nixpkgs.podman (podman)
A program for managing pods, containers and container images